1. Field of the Invention
The present invention relates to a braking system for use in an automotive vehicle and particularly to a hydraulic braking system comprising an anti-lock control apparatus.
2. Description of the Prior Art
In a conventional service braking system for an automotive vehicle, a hydraulic pressure generator which generates a braking pressure in response to depression of a brake pedal is used for supplying a prescribed braking pressure with the wheel brake cylinders operating the braking device mounted on each road wheel. There are two types of hydraulic pressure generators. One is a master cylinder which generates a braking pressure in response to depression of the brake pedal as an input to the brake fluid of a reservoir, what is called, a static hydraulic pressure generator. Another is a dynamic hydraulic pressure generator which has a power source and generates a braking pressure regulated in response to depression of the brake pedal as an input to the hydraulic power pressure of the power pressure source.
As to the dynamic hydraulic pressure generator, in recent years, with employment of a hydraulic booster, namely a booster which actuates the master cylinder by the hydraulic power pressure supplied from the power pressure source in response to depression of the brake pedal, the hydraulic output pressure of this hydraulic booster is directly applied to the wheel cylinders of the road wheels. With popularization of a device for preventing a slip due to locking of the road wheels in braking operation, i.e., an anti-lock control apparatus, it has been proposed to use the hydraulic output pressure of the hydraulic booster as a control pressure of the anti-lock control apparatus, namely to use the hydraulic booster as a supplementary pressure source, as disclosed in Japanese Patent Publication No. 56-10219 published in 1981.
On the contrary, the art which prevents the pressure change of the hydraulic booster in anti-lock control operation is disclosed in Japanese Patent Laid-Open Publication No. 62-166150 published in 1987. Namely, a regulator, as a supplementary pressure source provided in the system, regulates the hydraulic output pressure of the power pressure source in response to the hydraulic output pressure of the master cylinder and outputs the regulated pressure. Furthermore, the changeover valves which apply the hydraulic output pressure of the regulator to the wheel cylinders in the anti-lock control operating condition and block the introduction of the hydraulic output pressure of the regulator into the wheel cylinders in normal condition are disposed between the anti-lock control valves and the master cylinder.
In the above related art, the orifice means for restricting fluid communication is disposed in the hydraulic conduit between the supplementary pressure source and the wheel cylinders in order to be indicative of the rising speed of the hydraulic pressure of the wheel cylinders in the anti-lock operation. This orifice means is generally incorporated in the solenoid valves such as the flow control valve shown in the above Japanese Patent Laid-Open Publication No. 62-166150, and the restriction (orifice) is not only formed in the restriction side but also is formed in the free communication side in general and furthermore a small restriction (orifice) is formed in the anti-lock control valves which are position in the master cylinder side in regard to the flow control valves.
As mentioned above, these flow control valves and the anti-lock control valves are connected to the master cylinder via the changeover valves.
Accordingly, the hydraulic output pressure of the master cylinder is supplied to the wheel cylinders via the anti-lock control valves and the flow control valves in the conventional braking operation, and the rising speed of the hydraulic pressure of the wheel cylinders is dropped by the restriction (orifice) of these valves and there is in danger of the generation of a bad brake feeling. In order to solve this drawback, it is thought to provide parallel bypass valves in regard to the anti-lock control valves and the flow control valves. In this bypass valve system, however, the disposition of the hydraulic conduit is complicated and the braking system becomes large. It has also been thought to form the restriction (orifice) in only the passage of the anti-lock control side of the changeover valves which are positioned in the master cylinder side in regard to the anti-lock control valve and the flow control valve without forming the restriction (orifice) in the anti-lock control valves and the passage of the flow control valve in the normal condition. In this arrangement, however, the changeover valves become large in mass, and the many adjusting factors exists among the valves so that it is difficult to design this system. Furthermore, the manufacturing cost is increased.